Embodiments herein generally relate to skew correction performed in multi function printing/copying devices. “Skew” refers to an accidental, usually slight, rotation of an image that occurs due to rotation of an image bearing substrate relative to a device such as a scan head. Most document feeders in scanners and copiers, especially high-speed document feeders, are susceptible to at least some degree of skew. Skew results in an image that is slightly rotated, compared to the original image. Thus, a slightly rotated image is subsequently output by being printed on a substrate, displayed on a screen, and/or the like.
In various reproduction systems, including Xerographic printing systems, the control and registration of the position of imageable surfaces such as photoreceptor belts, intermediate transfer belts, if any, and/or images on such imageable surfaces, and the control and registration of images transferred to and developed on a substrate, such as for example, a sheet of paper, involve both initial and process control methods.
To adjust the registration of images on either or both axes, i.e., the lateral axis and/or the process direction axis, relative to the image bearing surface and to one another, includes adjusting the position or timing of the images being formed on the image bearing surface. That may be done, for example, by controlling the raster output scanner (ROS) imaging system or of any other included latent or visible image forming systems.
Various systems and methods have been developed to control registration of image on paper after an initial registration has been made. Examples of such registration systems include those shown and described in U.S. Pat. Nos. 5,821,971; 5,889,545; 6,137,517; 6,141,464; 6,178,031; and 6,275,244, the subject matter of each patent incorporated herein by reference in its entirety. For example, U.S. Pat. No. 5,642,202, the subject matter of which is incorporated herein by reference in its entirety, discloses a process for initial registration calibration of a printing system including a printer and a master test image document printed by the printer.
There are a number of sources of image on sheet (IOS) or image on paper (IOP) registration errors which may be addressed, including process magnification, lateral magnification, lateral margin shifts, process margin shifts, paper skew and/or imager skew. Process magnification is the magnification of the image in the process direction, i.e., the direction in which the substrate onto which the image is transferred and developed moves through the image transfer and developing apparatus. Lateral magnification is the magnification of the image in the lateral direction, i.e., in the direction substantially perpendicular to the process direction. Paper skew is the angular deviation of the longitudinal axis of the substrate in the process direction and/or the angular deviation of the lateral axis of the substrate perpendicular to the process direction. Imager skew is the angular deviation of the raster output scanner scan lines from the process direction or a line normal to the process direction.
The lateral margins are the spaces between each edge of the image transferred to and developed on the substrate and each adjacent edge of the substrate which is substantially parallel to the process direction. The process margins are the spaces between each edge of the image transferred to and developed on the substrate and each adjacent edge of the substrate which is substantially perpendicular to the process direction. It should be noted that, in many Xerographic image forming devices, each image is exposed successively by one or more raster output scanner imagers. Each raster output scanner has a start of scan (SOS) sensor and an end of scan (EOS) sensor. These sensors, i.e., the start of scan (SOS) and end of scan (EOS) sensors, along with the delay before the first pixel is imaged after the start of scan occurs, and the associated timing of when the start of scan occurs, establish the lateral and process margins of a latent image which is to be developed and transferred to a substrate.
Because the effects of these possible image on sheet or image on paper registration errors are interrelated, conventional image on sheet or image on paper setup/calibration procedures first requires correcting for any paper skew and imager skew errors, then correcting for any lateral and process magnification errors, and then correcting for any lateral and process margin errors.
Each correction step may involve multiple iterations of printing and measuring test images and adjusting imaging system parameters before registration error magnitudes are reduced to acceptable levels. U.S. Pat. No. 4,627,721, the subject matter of which is incorporated herein by reference in its entirety, discloses automatic adjustment of optical components in an optical scanning system after a technical representative has visually inspected sample copies of a test pattern and entered adjustment numbers at a control console. In one specific embodiment, one sample copy is compared by the technical representative with the test pattern to adjust the magnification setting and a sequence of a set of five copies are produced to allow coarse and fine adjustments to the focus. U.S. Patent Publication 2003/0133000, incorporated herein by reference, discloses a system that uses an initial set of measurements to determine and reduce each of a number of images on paper registration errors in a single operator step.
U.S. Patent Publication 2003/0128401, incorporated herein by reference, discloses a method of correcting output image squareness in a laser printer system having a Raster Output Scanning (ROS) device with a pivotal mounting. The method includes printing a test pattern that includes a plurality of markers printed in a predetermined geometric arrangement and calculating a skew angle based on the measured distances, preferably using the Law of Cosines. The ROS device is then automatically rotated by an amount sufficient to correct the skew angle. The ROS rotation is performed automatically by an adjustment system connected to the ROS. The method optionally includes a scanner for scanning the test pattern image and making the measurements automatically via a program configured to analyze the scanned image.